The present application is related to commonly-owned co-pending application Ser. No. 09/834,421 entitled xe2x80x9cCIRCUITS AND METHODS FOR PROVIDING A CURRENT REFERENCE WITH A CONTROLLED TEMPERATURE COEFFICIENT USING A SERIES COMPOSITE RESISTORxe2x80x9d filed on the same data herewith, which application is incorporated by reference in its entirety.
1. The Field of the Invention
The present invention relates to the field of bandgap voltage reference circuits. In particular, the present invention relates to circuits and methods for providing a bandgap voltage reference using a series composite resistor and without requiring the use of an operational amplifier.
2. Background and Related Art
The accuracy of circuits often depends on access to a stable bandgap voltage reference. Accordingly, numerous bandgap voltage reference circuits have been developed. Typically, conventional bandgap voltage reference circuits require an operational amplifier. However, operational amplifiers are often a significant source of error due to their intrinsic offset voltage. Accordingly, bandgap voltage reference circuits that use operational amplifiers may be too inaccurate for some applications.
Some conventional bandgap voltage reference circuits correct for this error by using more elaborate operational amplifiers with low offset voltage or fairly complex circuitry to minimize the effect of the operational amplifier offset voltage. While such circuits do indeed provide fairly accurate bandgap reference voltages, these circuits are larger due to the operational amplifier and associated correcting circuitry. Thus, these circuits may occupy significant chip real estate. In addition, these circuits may also be costly to fabricate and have higher power requirements due to the complex design.
Accordingly, what is desired is a bandgap voltage reference circuit that is small, yet accurate, and that is suitable for low power applications.
In accordance with the present invention, what is described is the structure and operation of a circuit that provides a stable bandgap voltage reference. The circuit provides an accurate bandgap voltage reference without using an operational amplifier. Thus, the use of an elaborate low-offset operational amplifier or a complex correcting circuit normally associated with standard operational amplifiers is also avoided. Surprisingly, this is accomplished by using a composite resistor in both a current source and a voltage reference leg of the bandgap voltage reference circuit. As this result is far from obvious, the description includes a detailed proof illustrating why such a circuit does indeed result in an accurate bandgap reference voltage.
The bandgap voltage reference circuit includes a current source and a bipolar transistor that are coupled together such that current from the current source passes through the bipolar transistor to a low voltage source such as ground. A composite resistor is coupled in series between the current source and the bipolar transistor. The composite resistor of this voltage reference leg of the circuit is composed of at least two component resistors. Each resistor may be fabricated using standard CMOS processes so that the temperature coefficient of the composite resistor as a whole may be customized to the operating conditions of the bandgap voltage reference circuit. In one embodiment, the component resistors are coupled in series between the current source and the bipolar transistor.
The temperature coefficient of the composite resistor may be designed so as to generate a stable bandgap voltage reference for temperature variations within the operating range of the circuit. Accordingly, the circuit also provides a bandgap voltage reference that is relatively stable with normal supply voltage fluctuations.
The current source includes a relatively high voltage source and a relatively low voltage source. The current source includes two potential current paths from the high voltage source to the low voltage source. These potential paths are called a reference leg and a mirror leg. The reference leg includes a number of MOS transistors coupled in series between the high voltage source and the low voltage source. The MOS transistors include a group of at least one PMOS transistor that is electrically closer in the series to the high voltage source. The MOS transistors also include a group of at least one NMOS transistor that is electrically closer in the series to the second voltage source. The reference leg also includes a series composite resistor that includes at least two component resistors that are coupled in series with each other between the high and low voltage sources. The series composite resistor is disposed on either side of the plurality of MOS transistors in series between the high and low voltage sources. The mirror leg is coupled with the reference leg so that current flowing through the reference leg is mirrored in the mirror leg.
If a PNP bipolar transistor is implemented in each of the mirror leg and the reference leg in the current source, then the PMOS transistors define a current mirror while the NMOS transistors share a common gate voltage. If an NPN bipolar transistor is implemented in each of the mirror leg and the reference leg, then the NMOS transistors define a current mirror while the PMOS transistors share a common gate voltage.
This current source provides a current that is relatively stable with supply voltage fluctuations. This allows the bandgap voltage reference circuit as a whole to provide a bandgap voltage reference that is relatively stable with supply voltage fluctuations.
In one example, the composite resistor in the bandgap voltage reference leg of the circuit is a series composite resistor that is matched to the series composite resistor in the current source. For a given set of parameters, this provides a bandgap voltage reference of approximately 1.23 Volts with a downside curvature with temperature. However, this is by no means the only possible configuration for the composite resistor in the bandgap voltage reference leg. For example, by changing the temperature coefficient of the composite resistor at the bandgap voltage reference leg (by changing the size or configuration of the component resistors), the bandgap voltage reference may provide a different voltage with an upside curvature with temperature for the same CMOS process. In addition, the temperature coefficient of the composite resistors may be adjusted to offset first and second order variation of the bandgap voltage reference. This adjustment is often referred to as curvature correction.
The bandgap voltage reference circuit in accordance with the present invention has significant space advantages in that it builds upon an already useful circuit, the current source. The current source provides a current that is substantially stable with the temperature and may be useful for any circuit that requires a current reference. However, as will be explained in further detail in the following description, a reliable and accurate voltage reference circuit may be constructed by adding just two MOS transistors, a bipolar transistor and a composite resistor to the current source.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.